1. Field of the Invention
The present invention relates to a laser irradiation apparatus to irradiate an irradiation object with a laser beam and moreover to a method for manufacturing a semiconductor device with the use of the laser irradiation apparatus.
2. Related Art
In recent years, a technique to manufacture a thin film transistor (TFT) over a substrate has made a great progress, and application development to an active matrix display device has been advanced. In particular, a TFT formed using a poly-crystalline semiconductor film is superior in field-effect mobility (also referred to as mobility simply) to a TFT formed using a conventional non-single crystal semiconductor film, and therefore high-speed operation becomes possible when the TFT is formed using the poly-crystalline semiconductor film. For this reason, it has been tried to control a pixel by a driver circuit formed over the same substrate as the pixel, which was controlled conventionally by a driver circuit provided outside the substrate.
A substrate used in a semiconductor device is expected to be a glass substrate in terms of cost. However, the glass substrate is inferior in heat resistance and easy to change in shape due to the heat when the process temperature is high. Therefore, when the TFT using the poly-crystalline semiconductor film is formed over the glass substrate, a laser annealing method is employed to crystallize a semiconductor film formed over the glass substrate in order to prevent the glass substrate from changing in shape due to the heat.
Compared with another annealing method which uses radiation heat or conduction heat, the laser annealing has advantages that the processing time can be shortened drastically and that a semiconductor film over a substrate can be heated selectively and locally so that the substrate is hardly damaged thermally.
In general terms, when the semiconductor film is crystallized by the laser annealing method, a beam spot is shaped into linear or rectangular by an optical system. The mass productivity can be enhanced by performing the laser irradiation to the irradiation object effectively in such a way that the beam spot is moved relative to the irradiation object in two directions of a short-side direction and a long-side direction of the beam spot. The short-side direction means a direction parallel with a short side of a rectangular beam spot, and the long-side direction means a direction parallel with a long side of the rectangular beam spot.
In the laser annealing method, it is important to control the irradiation position of the laser beam to the irradiation object. Conventionally, in order to control the irradiation position of the laser beam accurately, a technique is employed in which a marker to be the standard is provided to the irradiation object and the irradiation position is controlled based on the marker by using image processor means including a CCD (Charge Coupled Device) camera, a computer, and the like (for example refer to patent document 1).
[Patent Document] Japanese Patent Laid-Open No. 2003-224084
However, although the image processor means such as the CCD camera determines the irradiation position of the laser beam once based on the marker provided to the irradiation object, the irradiation position is gradually displaced from the region to be irradiated with the laser beam while repeating the laser irradiation. This is because a moving device such as a stage to be used in the laser irradiation does not have the absolute accuracy.
Although this displacement is very small just after the start of the irradiation, the displacement increases with the repeated movement of the stage. For example, when the laser irradiation position is controlled on a large substrate having a length of 1 m or more on a side, the displacement due to the movement of the stage appears remarkably and this leads to a significant problem.
For example, when an amorphous semiconductor film formed over a substrate is crystallized by the laser irradiation, a problem causes that a non-crystallized region or a repeatedly-irradiated region is formed due to the displacement of the laser irradiation position. It is considered that when TFTs are manufactured using the semiconductor film crystallized thus, the electrical characteristic varies between the TFTs and the reliability is lowered.
Moreover, when the amorphous semiconductor film is crystallized by the laser irradiation, the irradiation object is moved relative to the laser beam at a constant speed in two directions in order to grow the crystal in the scanning direction of the laser beam. In this case, when the irradiation position of the laser beam is controlled in two directions by the image processor means including a CCD camera, it takes much time to control the irradiation position takes much time, and therefore the processing speed becomes slow. When the moving speed of the stage is fast, highly sophisticated image processor means is necessary to control the irradiation position of the laser beam.